The later part of the hearing is about the science of marijuana and impairment. The editorial doesn’t get into that—there is only so much you can cover in 325 words—but the testimony was fascinating.

First on the science was Paul Armentano, deputy director of the National Organization to Reform the Marijuana Laws—i.e., a marijuana advocate. He said the effect of marijuana peaks about 40 minutes after smoking, though the psychoactive chemical THC peaks in the blood much sooner than that, and that performance (presumably including driving) is “relatively normal by two to three hours from inhalation.” He said experienced users learn how to cope with it, that the effect on their driving is “relatively small,” and that drivers on marijuana typically drive slower and are less aggressive.

Most fascinating was the statement of Marilyn Huestis, a senior marijuana toxicologist at the National Institute on Drug Abuse in Bethesda, Md. She said she had had participated in one study of chronic smokers, defined as people who smoke marijuana every day (Karshner, et.al., 2008) and had another study accepted for publication but is not yet on line. Her studies showed for occasional smokers, THC dissipates in the blood rapidly, but for chronic smokers it dissipates rapidly to a certain point, then falls slowly. This is because the body stores THC in the fat and chronic users “build up a very large body burden of THC in their system that’s slowly released over time.” She added later, “The brain is the fattiest tissue in the body.”

In this second study, she and two other scientists imaged the brains of chronic users and found that they had fewer receptors for memory, motor control, balance, hunger, perception, peripheral vision and body temperature control than in other people. “When you overstimulate the brain with frequent use,” she said, “the brain …reduces the number of receptors available to carry out normal functions.” In other words, “The brain changed.”

She had these smokers on the unit for 40 days, not smoking and slowly excreting the stored THC from their systems. She discovered that over time, regarding these receptors, the brain changes back.

In the meantime, though, there is an effect on the ability to drive. Her study found some impairment in the high-use group 21 days after they had last smoked “compared with occasional cannabis users.” None of these impaired chronic users had 5 nanograms of THC per milliliter of blood after the first 24 hours, or anything like that much. The drug wasn’t stored in the blood. It was “in the brain,” she said.

What that seems to mean is that an adult brain can take occasional use of marijuana without these long-term effects. Chronic use is not so good. And for most people, that is not so different from the same general rule they should have for alcohol.